The present invention relates to improved magnetically tunable filters, and more particularly to planar slotline microwave filters.
Besides the obvious use as bandpass or bandstop filters, microwave filters have been used in the field of medicine to study the characteristics of bodily parts which are comprised of mostly aqueous solutions. Recently, there has been considerable interest in characterizing or identifying defective human heart cells using microwave techniques. A standard microwave technique which is often used for this and similar research in medicine is the Electron Paramagnetic Resonance Technique EPR. EPR attempts to use resonant cavities have proven ineffective for a variety of reasons. For example, the aqueous nature of the heart cell causes near total absorption of the microwave energy in the cavity. Hence, the cavity fails to resonate and a biological analysis of the microwave data cannot be made. Additionally, due to the necessity for the cavity to operate at one resonant frequency, response characteristics over a wide frequency range cannot be obtained. Much more useful information can be deduced, if microwave experiments can be performed over a wide frequency range.
Tunable band pass filters are known in which an input path and an output path are coupled by bulk ferri- or ferro-magnetic material. The tuning of the filter resonant frequency is achieved by applying a variable dc magnetic field to spherical ferri- or ferro-magnetic material. Such known filters are complicated and expensive to make, and while passing the signals of the pass band frequencies with little attenuation these filters often lack isolation between the input and output paths at frequencies other than the pass band of the filter. With the advent of the semiconductor chip, the microwave technology has shifted toward planar microwave circuitry. Accordingly, it is desirable to have a planar microwave filter wich can be fabricated using presently known semiconductor technology.